UDIDs are unique 40-character codes assigned to all iDevices with cellular connectivity. They were previously used primarily for app registration and tracking by third-party developers, but Apple stopped that practice this year.

AntiSec published a sample of 1,000,001 iPhone and iPad identifiers, but stripped out any identifying data and left just the Apple Device ID, Apple Push Notification Service DevToken, Device Name and Device Type data.

However, the FBI has now said: "The FBI is aware of published reports alleging that an FBI laptop was compromised and private data regarding Apple UDIDs was exposed.

"At this time there is no evidence indicating that an FBI laptop was compromised or that the FBI either sought or obtained this data."

The agency's press office said in an additional tweet: "We never had info in question. Bottom Line: TOTALLY FALSE"

In an earlier blog on Pastebin, AntiSec predicted that the FBI would play down the leak, adding: "FBI will, as usual, deny or ignore this uncomfortable thingie and everybody will forget the whole thing at amazing speed."

The group also claimed in the account that the laptop hacked was that of "Supervisor Special Agent Christopher K Stangl from FBI Regional Cyber Action Team and New York FBI Office Evidence Response Team".

Hacking collective Anonymous has since posted on its Twitter account: "FBI says there was no hack. That means either they're lying or they *gave* the information up to someone in #antisec. It's happened before.

"If it's the latter, watch out. They're looking to bolster the cred of someone within #antisec, and will announce arrests in 6-8 months."